This disclosure relates to methods and systems for processing Global Navigation Satellite System (GNSS) navigation data, and in particular, methods and systems for demodulating GNSS navigation data bits in conditions that can cause a Doppler error, such as a GNSS receiver operating under a poor clock condition.
In existing GNSS systems, navigation data contains position and/or time related information such as ephemeris, almanac, satellite time, etc. The navigation data is coded to binary bits and modulated on a GNSS carrier signal along with various pseudorandom codes. The navigation data is used by a GNSS receiver, for example, to perform various navigation capabilities, such as calculating position, velocity and time (PVT) of the receiver.
Some existing demodulation methods employed for GNSS signal processing, including non-coherent demodulation, can be sensitive to the accuracy of the carrier signal. Therefore, in order for the demodulation techniques to be precise, the GNSS signal needs to be well tracked. Consequently, in instances where the tracked carrier Doppler experiences a significant error, the performance of existing demodulation will degrade and the demodulated bits will potentially become inaccurate. As an example, a significant error to the carrier Doppler can be characterized by a Doppler error that is larger than 1/(47) Hz (i.e., T is a coherent integration time for correlation results). Therefore, it is desirable to compensate for Doppler error in demodulating a GNSS carrier signal.
The background description provided above is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.